Ultrasound imaging systems are finding increased popularity for use in conjunction with needle probes inserted into the body, for example for purposes of aspiration biopsy. Traditionally, it was held that the very high sonic impedance of the needles (traditionally stainless steel) made it difficult if not impossible to locate the needle precisely through the conventional pulse echo methods of ultrasound imaging systems. That is, sonic wavefronts passing through soft tissue, when they impinged upon the needle, would be scattered unpredictably and hence could not be precisely located in the field of the image. Hence, numerous prior art patents feature a needle entry either through the center of, or at least coaxial with the application of sonic energy into the patient. Only when the needle digresses from the normal would any signals show up in the system, and hence those prior art systems rely on absence of needle signal, rather than presence of needle signal, to indicate proper direction. Clearly, however, those systems do not give an adequate indication of needle depth, and are extremely inconvenient to use due to their necessity of applying the needle at the point of the transducer.
U.S. Pat. No. 4,249,539 to Vilkomerson et al., issued Feb. 10, 1981, and entitled "Ultrasound Needle Tip Localization System", addresses the problems attendant to the above-described prior art systems, and posits a solution based on location of a miniature transducer at the tip of the penetrating needle. This transducer, which functions essentially as a point source transducer and acts in addition to the external, ultrasound imaging system transducer, constitutes an active sonic receiver at the needle point, which is coupled to associated processing apparatus through electrical conduits within the needle, and hence is enabled to generate return signals from the needle point to the system transducer. Alternatively, it forms the basis for superposition of the needle point on the system image, based on the time of flight between the generation of an imaging pulse at the system transducer, and the receipt thereof at the point transmitter associated with the needle. In accordance with the teachings of the Vilkomerson et al. patent, once the needle is properly positioned, the point source transducer may be withdrawn from the needle, and aspiration biopsy, insertion of drugs or markers, or the like may be conducted through the needle in conventional fashion.
The systems and methods set forth in the Vilkomerson et al. patent do provide a viable, accurate approach to the localization of the needle probe. Such approach is not without its problems, however. First, a point source transducer for maintenance at the needle tip, and withdrawal through the needle, is not inexpensive, and in any event requires placement of an active electrical element deep within the body of the patient. Secondly, necessity to have the transducer penetrate human flesh requires sterilization, another inconvenient and expensive requirement. Finally, some forms of transducer adequate in size and configuration for applications in accordance with the Vilkomerson patent, for example those configured of polyvinylidine fluoride (PVF2), provide relatively low electrical signal levels, and hence necessitate relatively complex and expensive signal processing systems outside the body.
It is accordingly an object of the present invention to provide systems of the sort set forth in the Vilkomerson et al. patent, but which obviate the need for locating active electrical elements within the patient's body, and hence likewise obviate safety difficulties attendant thereto, and difficulties attendant to sterilization of such active elements.